1. Basic type
The numerical performance of Kotlin, as follows
- Double 64
- Float 32
- Long 64
- Int 32
- Short 16
- Byte 8
These are all the ways to define numbers, and our general way of writing is similar to JAVA
- Decimal: 10086
- Hex: 0x0F
- Binary: 0b00001011
- Long type: 10086L
- Float Type: 10086f
- Double type: 100.86
At the same time, there is an underscore definition on Kotlin, which can make the code more readable. Let's take a look
fun main(args: Array<String>) {
val oneMillion = 1_000_000
val creditCardNumber = 1234_5678_9012_3456L
val socialSecurityNumber = 999_99_9999L
val hexBytes = 0xFF_EC_DE_5E
val bytes = 0b11010010_01101001_10010100_10010010
print("$oneMillion \n")
print("$creditCardNumber \n")
print("$socialSecurityNumber \n")
print("$hexBytes \n")
print("$bytes \n")
}The numbers output by this code
Underscore does not affect our code like JAVA, but makes our code more readable
And in the JAVA platform, the concepts of == and === are similar to == and eques in our JAVA
val a: Int = 10000
// true
print(a === a)
val boxedA: Int? = a
val anotherBoxedA: Int? = a
// false
print(boxedA === anotherBoxedA)In this code, we can see that we define an Inta, and define two nullable Ints to assign a value to a, and the result is ===false, but if we
change ==, then the value is equal, == = is the same address, obviously we are two different variables, so false
In addition, in Kotlin, small types are not subclasses of large types, so there is no concept of coercion
such as this wrong example
val b: Byte = 1
val i: Int = b
This is an implicit conversion, but we can use an explicit conversion to achieve our needs
val b: Byte = 1
val i: Int = b.toInt()
print(i)Each number can achieve the desired conversion by displaying the conversion
Let's look at logical operations again
- shl(bits) – signed left shift (<< in Java)
- shr(bits) – signed right shift (>> in Java)
- ushr(bits) – signed right shift (>>> in Java)
- and(bits) – bit AND
- or(bits) – bit or
- xor(bits) – bitwise exclusive or
- inv() – bit not
Let's write an example of a demonstration
val a: Int = 3
//<< 1
print("${a shl 1} \n")
//>> 1
print("${a shr 1} \n")
//>>> 1
print("${a ushr 1} \n")
//and 1
print("and: ${a.and(5)} \n")
//or 7
print("or : ${a.or(5)} \n")
//xor 6
print("xor : ${a.xor(5)} \n")
//inv -4
print("inv : ${a.inv()} \n")
Let's take a look at Char, Char cannot exist as a single character or number, and needs to be declared with '' single quotation marks, such as
var a : Char = '1'Let's take a look at how to define an array
//定义items["1","2","3"]
val items = listOf<String>("1","2","3")
//指定大小，内容为空
val itemNulls = arrayOfNulls<Int>(10)
// 创建⼀个 Array<String> 初始化为 ["0", "1", "4", "9", "16"]
//val asc = Array(5, { i -> (i * i).toString() })
// 定义五个数 一直递增
val asc = Array(5, {i -> (i * i)})listOf directly declares, arrayOfNulls declares an empty array of fixed length Array defines an array of specified length, value
Look at the string again
for (c in "Hello") {
println("$c")
}This is our common string, which can contain escapes, and there is also a
val text = """
for (c in "foo")
print(c)
"""This is a native string, represented by three """. It does not contain escapes. It is what it is. Of course, there are a lot of spaces in this case.
val text = """
|for (c in "foo")
|print(c)
""".trimMargin()
print(text)We can | declare the current start bit, and trimMargin() removes spaces, the result
If you don't handle it like this, your output value will look like this
2. Package
I believe everyone knows the role of the bag
package com.liuguilin.kt_package
fun main(args: Array<String>) {
print(sum(1,3))
}
fun sum(a: Int, b: Int): Int {
return a + b;
}
3. Control flow: if, when, for, while
1.if expression
In kotlin, the usage of if is not limited to judgment, it will also return, so we have many ways of writing, such as
val a = 12;
//传统
if (a > 10) {
print(">")
} else {
print("<")
}
//表达式
if (a > 10) print(">") else print("<")
//表达式
var b = if (a > 10) 10 else 202.When expression
val today = 4;
when (today) {
1 -> print("春季")
2 -> print("春季")
3 -> print("春季")
4 -> print("夏季")
5 -> print("夏季")
6 -> print("夏季")
7 -> print("秋季")
8 -> print("秋季")
9 -> print("秋季")
10 -> print("冬季")
11 -> print("冬季")
12 -> print("冬季")
}
But we generally need an else ending, and we can use in to express
val today = 4;
when (today) {
in 1..3 -> print("春季")
in 4..6 -> print("夏季")
in 7..9 -> print("秋季")
in 10..12 -> print("冬季")
else -> print("error")
}Of course, you can also use is to determine the type, so I won't talk about it here.
3. For loop
var items = listOf(1,2,3);
//out 123
for(a in items) print(a)
//out 012
for (a in items.indices) print(a)
//out 0 , 1 1 , 2 2 , 3
for ((a,b)in items.withIndex()) print("$a , $b \n")4.While loop
var a = 5;
while (a > 3){
print(a)
a--;
}
do {
print(a)
a--;
}while (a > 3)4.Break and continue
Kotlin has three structured jump expressions:
- return. Defaults to returning from the function or anonymous function that most directly surrounds it.
- break. Terminate the loop that most directly surrounds it.
- continue. Continue with the next loop that most directly surrounds it.
Let's look at an example
var items = listOf(1, 3, 5, 7, 9)
for (a in items) {
if (a == 5) {
continue | break | return
}
print("$a \n")
}Here, an array is looped. If it is equal to 5, continue, break, and return are executed respectively. If it is break, return will jump out of the loop, which will
only print 1, 3 and continue will jump out of this loop, and will also print 1, 3, 7,9
